Stripes & Staples

Why are pyjamas primarily of the striped variety?

They replaced the plain white nightshirt in the 19th century, being a fashion import from the Subcontinent, but stripes?

I suppose the stripes are somewhat similar to the stripes on men's shirts, but if that was their intent, then the bottoms should be black or navy blue to resemble trousers.

Yes, tartan seems to have taken over more recently, but then why tartan? 

Stripes were prison uniform and I wonder if some enterprising prison uniform manufacturer in Victorian Britain saw an opening in the market by making all men look like convicts at night.

As for staples - why do some newspaper publishers use staples in their newspapers? Not only is it a waste of money, but it adds unnecessary metal to the paper recycling. Some, like the Observer, use staples in the main section but not in the sports section - probably to aid throwing the sport section away immediately, like I do. 

I much prefer using the sports section of the Observer for lighting the fire, as the pages are easy to extract and crumple up, whereas the main section requires a fight with the staples, which tears the paper into small bits.